1. Field of the Invention
The present invention deals with the manufacturing of Fexe2x80x94Ni alloys used in press molded flat mask cathode-ray tube (CRT) screens. Specifically, by selecting the type and concentration of specific additional elements in the alloy, and by controlling the conditions under which the hot rolling of the alloy is conducted, it is possible to maintain the low thermal expansion and drop-shock resistant characteristics of Fexe2x80x94Ni alloys while controlling crack formation during the manufacturing process.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
CRT displays show images on screen by directing electron beams emitted from electron guns onto a fluorescent material placed behind a glass panel. The direction of the electron beams are magnetically controlled by a deflecting yoke. Between the electron gun and the glass panel, a structure is mounted which divides the beams into pixel-point units and directs the beams to the proper fluorescent material. This structure is known as a xe2x80x9cmask.xe2x80x9d Two types of masks are used in CRT displays: shadow masks, which are formed into press molded material which have been etched with dots or slots; and aperture grills, which are constructed by tightly stretching (in a vertical direction) material which have been etched with vertical slots onto supporting frames. Both methods have advantages and disadvantages, and are both commonly used in the current market.
Much effort has been expended toward creating xe2x80x9cflat screenxe2x80x9d displays that will display the image on a flat surface. By xe2x80x9cflat screenxe2x80x9d we mean here a display screen that is nearly totally flat as opposed to the traditional curved screen. One of the biggest problems in making flat screens for CRT displays is how to construct shadow masks or aperture grills that are as flat as possible. There are difficult technical problems with each, but basically it is considered more difficult to construct a flat screen by pressing a shadow mask into a flat shape than to construct one through the xe2x80x9changingxe2x80x9d methods used in aperture grills. (e.g., see NIKKEI ELECTRONICS 1999.7.26 (No. 748) p. 128).
This is due to the fact that since shadow masks are constructed by forming press molded metal sheets, unlike with the xe2x80x9cframe-hangingxe2x80x9d method, the material needs to maintain shape through its own shape maintaining ability. Basically, what this means is that the mask needs to be curved in order to maintain its shape. Therefore it is difficult to maintain the shape in a nearly level state as required for a flat mask. The only way to overcome this problem is to improve the strength of the mask. The xe2x80x9cstrengthxe2x80x9d in question here is not the strength of the metal per se (as is measured by tensile testing, for instance), but the strength of the mask after the CRT has been put together as determined by whether or not the shape of the mask is affected when the entire CRT is subjected to shock. Specifically, the CRT is dropped from a certain height to test whether the shape of the mask is affected. The development of masks that are resistant to deformation from this type of shock (drop-shock resistant) is crucial for the production of flat-mask CRTs. It is known that the Young""s modulus and proof stress of the mask material most strongly affects the evaluation of drop-shock resistance.
In addition, flat masks are required to have excellent doming properties. That is, as the masks are made flatter, the angles at which the electron beams hit the mask at the four corners become sharper. This means that slight misalignments caused by thermal expansion of the mask cause the beam to be misaligned, leading to color distortion in the displayed image. Therefore there is a need to develop masks with much lower thermal expansion than conventional masks.
The basic material used for shadow masks has been Fexe2x80x94Ni (33-37%) alloy with added Mn. The ease with which Fexe2x80x94Ni alloy can be hot worked is greatly affected by the amount of S contained in the alloy. The greater the amount of S, the less workable the alloy is. In order to counter the effect of S within the alloy on its workability, it is effective to add Mn to the alloy, causing the S within the alloy to chemically combine with the Mn to form MnS. In general, the greater the ratio of Mn to S within the alloy, the greater the improvement in hot workability; a ratio of at least 50-100 Mn/S is needed. Mn also serves as a deoxidization agent. However the addition of Mn increases the thermal expansion coefficient. For flat masks, an average thermal expansion coefficient of below 12xc3x9710xe2x88x927/xc2x0 C. at 30-100xc2x0 C. must be achieved.
Thus, in a press molded shadow mask, much lower thermal expansion and higher drop-shock resistance is required than in conventional masks. Therefore, there has been previously disclosed in patent application JP2000-192249, an Fexe2x80x94Ni alloy designed to decrease the amount of added Mn and obtain a high proof stress, namely an Fexe2x80x94Ni alloy to which Co is added in appropriate amounts as needed in relation to the amount of Ni, with Nb, Ta and Hf added as appropriate, while limiting the amount of impurities in the alloy. Such alloy contains 33-37% Ni; 0.001xe2x80x940.1% Mn; optionally 0.01-2% Co; and at least one of (1) 0.01-0.8% Nb; (2) 0.01-0.8% Ta; and/or (3) 0.01-0.8% Hf, with the total of Nb, Ta, and Hf being in the range of 0.01-0.8%.
However, although the above-mentioned alloy possesses characteristics making it suitable for use in flat masks, because the amount of Mn is limited to the low level of 0.001-0.1%, and despite limiting the amount of S in the alloy to below 0.002%, such alloy has a propensity toward developing edge and surface cracks while undergoing hot rolling during manufacture. In addition, it has been noted that the inclusion of Nb, Ta and Hf, which increases drop-shock resistance, also contributes to diminished workability, leading to an increase in edge and surface crack formation.
The purpose of the current invention is to find the conditions for hot rolling which will limit the development of edge and surface crack formation in the aforementioned alloy during hot rolling.
Having analyzed conditions leading to decrease of edge and surface crack formation in the aforementioned alloy, it has been concluded that the distortion rate within each pass of hot rolling is especially crucial, and furthermore that the heating conditions prior to hot rolling and the temperature at the completion of hot rolling are also important.
The present invention provides a method of manufacturing an improved Fexe2x80x94Ni alloy consisting of 33-37% Ni; 0.001-0.1% Mn; optionally 0.01-2% Co; and at least one of (1) 0.01-0.8% Nb; (2) 0.01-0.8% Ta and/or (3) 0.01-0.8% Hf, with the total of Nb, Ta and Hf being in the range of 0.01-0.8%, and the remainder being Fe and unavoidable impurities. The method comprises subjecting the alloy to a hot rolling process wherein the rate of distortion during each pass of hot rolling is below 70/second resulting in the alloy having a reduced rate of crack formation during the hot rolling process as well as high drop-shock resistance and low thermal expansion after the hot rolling process. Preferably the main impurities in the alloy are limited to maximums of 0.01% C, 0.02% Si, 0.01% P, 0.01% S and 0.005% N.
Also preferred as part of the manufacturing process of this invention is heating the alloy at a temperature of 1000 to 1300xc2x0 C. for 0.5 to 10 hours before hot rolling, and performing the final pass of hot rolling at a temperature of the alloy above 600xc2x0 C.